Deformation processes and rheology of pyroxenites under lithospheric mantle conditions

We combined microstructural observations and high-resolution crystallographic preferred orientation (CPO) mapping to unravel the active deformation mechanisms in garnet clinopyroxenites, garnet–spinel websterites, and spinel websterites from the Beni Bousera peridotite massif. All pyroxenites display microstructures recording plastic deformation by dislocation creep. Pyroxene CPOs are consistent with dominant slip on [001]{110} in clinopyroxene and on [001](100) or [001](010) in orthopyroxene. Garnet clinopyroxenites have however high recrystallized fractions and finer grain sizes than spinel websterites. Recrystallization mechanisms also differ: subgrain rotation dominates in garnet clinopyroxenites, whereas in spinel websterites nucleation and growth also contribute. Elongated shapes and strong intracrystalline misorientations suggest plastic deformation of garnet, but CPOs are weak. Clinopyroxene porphyroclasts in spinel websterites show deformation twins underlined by orthopyroxene exsolutions. Thermodynamic calculations indicate that garnet clinopyroxenites deformed at 2.0 GPa and 950–1000 °C and spinel pyroxenites at 1.8 GPa and 1100–1150 °C. The lower temperatures may explain the faster work rates implied by the finer grained microstructures in garnet clinopyroxenites. Greater stresses may have also reduced the competence contrast between garnet and pyroxene in the garnet pyroxenites and, at the outcrop scale, lowered the competence contrast between pyroxenites and peridotites, favoring mechanical dispersion of pyroxenites in the cooler lithospheric mantle.     

Strength criterion for cross-anisotropic sand under general stress conditions

By incorporating the fabric effect and Lode’s angle dependence into the Mohr–Coulomb failure criterion, a strength criterion for cross-anisotropic sand under general stress conditions was proposed. The obtained criterion has only three material parameters which can be specified by conventional triaxial tests. The formula to calculate the friction angle under any loading direction and intermediate principal stress ratio condition was deduced, and the influence of the degree of the cross-anisotropy was quantified. The friction angles of sand in triaxial, true triaxial, and hollow cylinder torsional shear tests were obtained, and a parametric analysis was used to detect the varying characteristics. The friction angle becomes smaller when the major principal stress changes from perpendicular to parallel to the bedding plane. The loading direction and intermediate principal stress ratio are unrelated in true triaxial tests, and their influences on the friction angle can be well captured by the proposed criterion. In hollow cylinder torsional shear tests with the same internal and external pressures, the loading direction and intermediate principal stress ratio are related. This property results in a lower friction angle in the hollow cylinder torsional shear test than that in the true triaxial test under the same intermediate principal stress ratio condition. By comparing the calculated friction angle with the experimental results under various loading conditions (e.g., triaxial, true triaxial, and hollow cylinder torsional shear test), the proposed criterion was verified to be able to characterize the shear strength of cross-anisotropic sand under general stress conditions.     

Some kinetics of bronzite orthopyroxene dissolution

The kinetics of bronzite orthopyroxene dissolution were investigated in HClKCl solutions having a total concentration of 0.1 M, over the pH range 1–4.5, at temperatures between 42 and 1°C. Dissolution of the pyroxene was incongruent and followed a parabolic rate law. The activation energy of the reaction is 10.5 ± 0.6 kcal mole^?1. The rate dependence on hydrogen ion activity is one-half order. The rate of dissolution is unaffected by substitution of sodium for potassium, or sulfate or nitrate for chloride anion, or by addition of citrate or acetate ions. However, traces of fluoride increase the dissolution rate. The rates observed are one to two orders of magnitude slower than those previously reported by Luceet al. (1972) for dissolution of enstatite.     

Reactions involving hydration of orthopyroxene in anorthosite-gabbro

Hydration of orthopyroxene (En₆₀) in contact with plagioclase (An₃₆) leads to oronas of the type orthopyroxene-cummingtonite-edenite + quartz-edenitic hornblende. The compositions of the minerals involved indicate that kyanite needles in the adjacent plagioclases form as part of the reaction: orthopyroxene (En₆₀) + plagioclase (An₃₆) + H₂O→ cummingtonite + edenite/edenitic hornblende + plagioclase (< An₃₆) + kyanite + quartz. The reaction proceedings consumption first of the orthopyroxene and later of the cummingtonite to produce a corona structure consisting of a core of intergrown edenite and quartz surrounded by edenitic hornblende. Corona evolution of this type is controlled by diffusion.     

Electron microscopical study of garnet exsolution in orthopyroxene

By means of electron-optical methods the structure and orientation of lamella-like garnet exsolutions in an orthopyroxene from the eclogite of Gilsberg, Saxony Granulite Complex, are investigated. It can be shown that besides the macroscopic and microscopic submicroscopic exsolution lamellae also exist; they have widths up to nearly 5 nm. The intergrowth conditions are such shat (100)_op is parallel to {100}_gr and [001]_op parallel to 〈110〉_gr. The lamellae are coherent, but sometimes extended dislocation arrangements are found in microscopic lamellae which are caused by a deformation. These investigations point to formation of the rock as a result of a solid state reaction under high pressure conditions with subsequent diffusive-dissociative exsolution of garnet from primary pyroxene.     

Exsolution of Ca-clinopyroxene from orthopyroxene aided by deformation

Monoclinic calcium-poor shear-transformation lamellae and calcium-rich exsolution lamellae occur parallel to (100) in orthopyroxene. The formation of both structures from an orthopyroxene host involves a shear on (100) parallel to [001], with additional cation exchange in the exsolution case. The shear transformation involves a macroscopic simple shear angle of 13.3° (shear strain of 0.236) and produces a specific a-axis orientation with respect to the sense of shear; we have found that this orientation dominates in exsolution lamellae in kinked orthopyroxene, where the sense of shear is known. In undeformed orthopyroxene, there is generally no preferred sense of orientation of the monoclinic a axes. We advance a specific model for exsolution involving nucleation and growth by shear transformation combined with cation exchange, thus circumventing the classical nucleation barrier and permitting exsolution at lower solute supersaturations.     

Electron petrography of exsolution in an enstatite-rich orthopyroxene

Lattice defects along (100) planes in an orthopyroxene, En_92.8-Fs_7.0-Wo_0.2, have been observed in the transmission electron microscope. From an analysis of the electron diffraction contrast at these defects it is proposed that the defects are regions of iron and possibly calcium enrichment. The defects are completely coherent with the lattice of the matrix and appear to be metastable or transition zones. They do not have any direct relationship with the spinodal decomposition proposed for the exsolution textures in some of the lunar pyroxenes of Apollo 11 and 12. Possible cationic ordering within these zones is discussed and the effects of such ordering on magnetic properties are considered.     

A transmission electron microscope study of naturally deformed orthopyroxene

A model is proposed for the production of clinopyroxene lamellae in orthopyroxene by a dislocation mechanism based on simple shear. Four possible shears are described. Two shears apply to each subcell of orthopyroxene, one with =13.3° in one sense, the other with =18.3° in the opposite sense.The senses of shears of the same magnitude in adjacent cells are also opposite. All shears produce the same structure, but in two discrete orientations which can be distinguished from electron diffraction patterns. However, the operative shear cannot be uniquely determined from the diffraction patterns alone. The characteristics of the diffraction contrast fringes and associated partial dislocations observed by transmission electron microscopy in a naturally deformed orthopyroxene are shown to be consistent with the proposed model.     

Xenolith evidence for lithospheric melting above anomalously hot mantle under the northern Canadian Cordillera

A comparison of mantle xenolith suites along the northern Canadian Cordillera reveals that the xenoliths from three suites exhibit bimodal populations whereas the xenoliths from the other four suites display unimodal populations. The bimodal suites contain both fertile lherzolite and refractory harzburgite, while the unimodal suites are dominated by fertile lherzolite xenoliths. The location of the three bimodal xenolith suites correlates with a newly discovered P-wave slowness anomaly in the upper mantle that is 200 km in width and extends to depths of 400–500 km (Frederiksen AW, Bostock MG, Van Decar JC, Cassidy J, submitted to Tectonophysics). This correlation suggests that the bimodal xenolith suites may either contain fragments of the anomalously hot asthenospheric mantle or that the lithospheric upper mantle has been affected by the anomalously hot mantle. The lherzolite xenoliths in the bimodal suites display similar major element compositions and trace element patterns to the lherzolite xenoliths in the unimodal suites, suggesting that the lherzolites represent the regional lithospheric upper mantle. In contrast, the harzburgite xenoliths are highly depleted in terms of major element composition, but their clinopyroxenes [Cpx] have much higher incompatible trace element contents than those in the lherzolite xenoliths. The major element and mildly incompatible trace element systematics of the harzburgite and lherzolite xenoliths indicate that they could be related by a partial melting process. The lack of textural and geochemical evidence for the former existence of garnet argues against the harzburgite xenoliths representing actual fragments of the deeper anomalous asthenospheric mantle. Furthermore, the calculated P-wave velocity difference between harzburgite and lherzolite end-members is only 0.8%, with the harzburgites having higher P-wave velocities. Therefore the 3% P-wave velocity difference detected teleseismically cannot be produced by the compositional difference between the lherzolite and harzburgite xenoliths. If temperature is responsible for the observed 3% P-wave velocity perturbation, the anomalous mantle is likely to be at least 200 °C higher than the surrounding mantle. Taken together these data indicate that the refractory harzburgite xenoliths represent the residue of 20–25% partial melting of a lherzolite lithospheric mantle. The incompatible trace element enrichment of the harzburgites suggests that this melting was accompanied by the ingress of fluids. The association of the bimodal xenolith suites with the mantle anomaly detected teleseismically suggests that anomalously hot asthenospheric mantle provided both the heat and volatiles responsible for the localized melting and enrichment of the lithospheric mantle. Received: 16 May 1997 / Accepted: 25 October 1997     

不同环剪条件下三峡库区童家坪滑坡滑带土强度特性

研究不同环剪条件下库岸堆积层滑坡滑带土强度特性对滑坡稳定性评价具有重要意义.针对目前在库岸堆积层滑坡滑带土力学特性方面研究薄弱的问题,以三峡库区童家坪滑坡滑带土为研究对象,采用ARS-E2环剪仪开展了不同剪切速率下的剪切试验,研究了等速剪切、加速剪切以及减速剪切作用下滑带土强度变化特征.试验结果表明:滑带土试样在恒定的...     

真三轴状态下砂土的强度参数

基于SMP强度准则和三剪切角理论,推导了一般应力状态和轴对称状态下砂土峰值内摩擦角间的转换公式,并研究了中主应力比的影响。考虑到轴对称压缩状态和轴对称拉伸状态所得峰值内摩擦角间的差异,对所得公式进行修正。将修正前后公式的预测结果与真三轴试验结果对比表明,若要合理描述砂土在一般应力状态下的强度特性,应当考虑三轴拉伸和三轴压缩状态下强度参数间差异的影响。     

岩石在循环荷载作用下的强度及变形特征

本文着重研究了岩石在循环荷载作用下的强度及变形特征。实验结果表明,岩石在循环荷载作用下的强度低于其静力强度,而这个循环疲劳强度极限与振幅、频率有关;岩石在循环荷载作用下的变形特征类似于静力蠕变特征。     

冀东地区变质岩中斜方辉石的研究

通过对冀东地区３４个斜方辉石榈的研究得出结论：斜方辉石由早、晚两期变质作用形成。斜方辉石的化学成分变化主要与变质作用的条件及岩中共存矿物种类有关，同时也受原岩成分的控制。据温压计的估算结果，早期变质作用的温度为８１７－９３７℃、压力为０．９９－１．１１ＧＰａ；晚期变质作用的温度为７４７－８３４℃、压力为０．９５ＧＰａ。     

The solubility of alumina in orthopyroxene coexisting with garnet

The solubility of alumina in complex orthopyroxenes crystallised in equilibrium with garnet has been determined over the pressure-temperature range 8–30 Kb and 800–1250° C. The results are in good agreement with predictions made using the simple thermodynamic model of Wood and Banno (1973). The model has been refined using a combination of the new data on orthopyroxenes of about En₆₀Fs₄₀ (with variable calcium content) and previously published data on more magnesian systems. Given analyses of coexisting Orthopyroxene and garnet in natural assemblages it is possible to calculate a P-T curve for the rock which should, in most cases, be accurate to within 2 or 3 kbar.     

扭转荷载下冻土的强度特性

对冻结黄土的一系列扭转试验结果表明；冻土受纯剪时的抗剪强度与其抗压，抗拉强度具有相同的规律，其抗剪强度随温度降低和破坏剪应变的减小而增大，同时也随扭速的加快而呈而线性递增，中还给出了各影响因素的相关分析式。     

Hydrogen incorporation in orthopyroxene: interaction of different trivalent cations

The incorporation of hydrogen into ferrosilite, Fe-bearing enstatite and orthopyroxene containing different trivalent cations (Cr³⁺ and Al³⁺, Cr³⁺ and Fe³⁺) was investigated experimentally at 25 kbar. Hydrogen concentration was determined by FTIR-spectroscopy on oriented crystal sections and by secondary ion mass spectroscopy, whereas Mößbauer spectroscopy and optical spectroscopy were used to characterise the valence state of Fe in orthopyroxene. Results suggest that hydrogen incorporation in ferrosilite is achieved by a similar mechanism as in pure enstatite. In Cr-bearing samples, however, hydrogen incorporation is reduced by the presence of other trivalent cations by an increased tendency to form Tschermaks substitutions, e.g. Si _T ⁴⁺ + Mg _M1 ²⁺ ? Al _T ³⁺ + Cr _M1 ³⁺ . Thus, hydrogen solubility in natural orthopyroxenes from the Earth’s mantle, containing significant amounts of Cr³⁺, Al³⁺, and Fe³⁺, may be much more limited than expected from their trivalent cation content, as a large fraction of the trivalent cations does not participate in H-incorporating reactions as 2 Mg _M1 ²⁺ ? M _M1 ³⁺ + V_M1 + H _i ⁺ .     

Bonding in silicates: an assessment of bonding in orthopyroxene

The molecular orbital and crystal field theories are compared and their applicabilities to bonding in silicates are discussed, A molecular orbital bonding model is favoured for orthopyroxene since bonding in both tetrahedral and non-tetrahedral sites within the crystal structure can be described and accommodated by the theory. It is suggested that Fe located in M₂ sites has more electron delocalisation (or covalence) associated with it than Fe located in the M₁ site. Order-disorder phenomena in orthopyroxene are discussed in relationship to Mueller's (1970) two-step exchange mechanism, whereby the first step corresponds to vacancy activation and the second step to exchange of Fe between an ‘octahedral’ site and a vacancy.